The invention relates to fasteners and fastener making tools in general, and more particularly to improvements in threaded fasteners and to improvements in tools for making threaded fasteners. Still more particularly, the invention relates to improvements in external and/or internal threads of fasteners and threaded tools which can be utilized with advantage as osteosynthetic force transmitting members, osteosynthetic accessories and/or for many other purposes.
A standard fastener which is to take up unchanging tensional stresses and/or dynamic stresses (namely tensional stresses of varying magnitude) and which is provided with a standard internal and/or external thread (e.g., an ISO metric screw thread) will be capable of withstanding such stresses only if the magnitude of applied unchanging tensional stresses does not exceed a certain limit and only when the magnitude of dynamic stresses varies within a relatively narrow range. Such fasteners can constitute form-locking screws, bolts, pins, inserts, tubes and many others including so-called tapping screws which are intended to be driven into a part consisting of material which can yield in response to the penetration of a threaded shank or the like so that the external thread establishes in the part a complementary internal thread.
Important factors which determine the ability of threaded fasteners (such as screws and/or bolts of all kinds, threaded pins or posts, threaded inserts and/or internally and/or externally threaded tubular members) to withstand unchanging and/or variable tensional and other stresses are the dimensions and the configuration of external and/or internal threads. For example, the exact configuration of flanks which extend between the roots and the crests of internal or external threads will exert a great influence upon the ability of threaded fasteners to withstand constant (unchanging) and/or varying tensional stresses. This holds true regardless of whether the dynamic stresses vary gradually or at a relatively high frequency. It is particularly important to properly select the depth of the thread as well as the configuration of those portions of the flanks which are adjacent the root of the thread. Such parameters influence the strength and the useful life of the threaded fasteners.
The depth of the thread is normally selected by taking into consideration the characteristics of the material of the part or parts which are to receive the thread of the fastener, i.e., of the part or parts which are to be brought into mesh with the threaded fastener. The reason is that the strength of a threaded fastener can be caused to vary within a wide range by changing the depth of the thread. Any change of the depth of the thread necessarily entails a change of the minor diameter of the thread of an externally threaded fastener, and such change of the minor diameter can greatly influence the carrying or bearing capacity of the fastener. As a rule, changes of the minimal radius of curvature of the flanks adjacent the root of the thread are inversely proportional to the fatigue strength reduction or fatigue notch factor. Otherwise stated, the fastener is more likely to break in response to unchanging tensional stresses if the radius of curvature of those portions of the flanks which are immediately adjacent the root of the thread is reduced. Therefore, the radius of curvature of those portions of the flank of an ISO metric screw thread which are immediately adjacent the root of the thread cannot be reduced below a minimum value constituting a certain fraction of the pitch of the screw thread. On the other hand, the selection of a relatively large radius of curvature for those portions of the flanks of a thread which are immediately adjacent the root brings about the drawback that the extent of thread overlap is reduced which, in turn, entails the application of increased stresses to the load carrying flank of a thread. Accordingly, the maximum concentration of tensional stresses takes place adjacent the root of the thread, i.e., such maximum stresses are applied to that portion of the stress-bearing flank of a conventional thread which is immediately adjacent the root. A concentration of maximum tensional stresses upon that portion of the stress-bearing flank which is adjacent the root of the thread due to the aforediscussed fatigue notch factor takes place in each region of varying cross-section and is dependent upon the extent of variation of the cross-section.
Accordingly, and if the configuration of those portions of the flanks of a thread which are immediately adjacent the root were to be selected exclusively on the basis of theoretical considerations, the cross-sectional area of the thread immediately adjacent the root should change at a minimal rate. It has been found that such requirements are not met by heretofore known standard threads wherein the configuration of the flanks is determined exclusively on the basis of circular or trigonometric functions.
When the externally threaded part is a screw, a bolt or an analogous fastener, the fastener and its thread are subjected to the action of three different stresses, namely tensional stresses, bending or flexing stresses and rotational or shearing stresses. Shearing stresses will develop primarily while an externally threaded fastener is being driven home (tightened) and also while an externally threaded fastener is being loosened. In spite of the relative shortness of intervals during which an externally threaded fastener is subjected to shearing stresses, such stresses still exert considerable influence upon the ability of the fastener to stand all forces when in actual use. When a tapping screw which is not provided with a core removing hole is being driven into the material of a part which is to be provided with a complementary internal thread, the major part of the resistance which is offered to penetration of the tapping screw into such material is caused by the shank of the tapping screw, i.e., the shank which is in the process of penetrating into the material of a part to be provided with a tapped bore must displace the material which is in the way of the tip of the shank. As a rule, the extent of work which is to be performed by a tapping screw during penetration of its shank into the material of a plate, another osteosynthetic accessory or the like is directly proportional to the cross-sectional area of the central (unthreaded) portion of the shank. Thus, if the resistance of the material to penetration of the shank is to be reduced, the core diameter of the shank must be reduced accordingly. This results in pronounced concentration of stresses at the root, i.e., in the regions of the radially innermost sections of the flanks. This will be readily appreciated by bearing in mind that the material to be displaced resists the advancement of the thread (namely of its flanks), i.e., this also involves a certain amount of work which is necessary to displace the material around the shank of the advancing tapping screw. Such concentration of stresses at the root of the external thread on a tapping tool can be reduced by resorting to harmonic variation of the cross-sectional area at the root and/or by reducing the extent of so-called moulding work of the thread. The just mentioned reductions can be achieved by appropriate shaping of the flanks not only within but also outside of the pitch diameter. As used herein, the term pitch diameter is intended to denote the diameter of an imaginary cylinder whose longitudinal axis coincides with the axis of the thread and which would cut the thread at a height where the width of the thread and of the helical groove between the flanks would be equal. Reference may be had, for example, to pages 1537-1538 of McGraw Hill "Concise Encyclopedia of Science and Technology" (1984 Edition).
When a tapping or thread cutting screw is driven into a ductile plastic material, the advancing tip of the shank initiates a material flow in a direction toward the groove of the thread and more specifically toward the root. The resistance which the plastic material offers to penetration of the tip of the shank of an externally threaded tapping tool will decrease if the profiles of the flanks on the thread of the shank offer relatively low resistance to the flow of displaced plastic material into the helical groove and toward the root of the thread. Based on presently known ductility of plastic materials, the displaced ductile material is bounded by a rounded surface. Therefore, flanks having substantially flat or straight outlines are more likely to oppose the flow of displaced plastic material toward the root of a tapping screw or an analogous thread forming tool than harmonically concave flanks which enable the external thread to act not unlike a plowshare and to thus direct the displaced plastic material into the groove and toward the root of the external thread.
The stress upon an externally threaded screw or bolt or a like member is further dependent on the ability of the complementary (internal) thread of the part (e.g., a plate) into which the screw is driven to resist shearing stresses. The connection between the externally threaded member and an internally threaded part is more satisfactory if the internal thread fills or at least substantially fills the helical groove of the externally threaded member. In other words, by properly selecting the surfaces (flanks) which bound the helical groove of an externally threaded member, one can increase the extent to which the helical groove of the externally threaded member is filled with the material of the internally threaded part. An ideal situation will develop if the internal thread completely fills the helical groove of the externally threaded member and if the externally threaded member cooperates with the internally threaded part to develop radial stresses; this ensures that, when the member is subjected to axial or longitudinal stresses, the internal thread which completely or practically completely fills the helical groove of the externally threaded member offers a pronounced resistance to longitudinal displacement of the externally threaded member. In other words, the extent to which the externally threaded member resists axial stresses is not dependent solely upon the pitch diameter of the externally threaded member because such resistance is assisted by the internal thread of the part which mates with the externally threaded member. Still further, such resistance of the externally threaded member to axial stresses is enhanced by radial forces which are generated by the flanks of at least one of the mating internal and external threads. Consequently, the configuration of the radially outer sections of flanks bounding the external thread of a screw or a like member also influence the desirable characteristics of such member. The configuration of radially outer sections of the external thread (namely the sections which are adjacent the crest of the external thread) will influence the ability of the externally threaded member to withstand shearing stresses.
German patent application Serial No. 32 07 975 A1 filed by Richard Bergner GmbH & Co. (published Sep. 15, 1983) discloses a thread cutting screw which is to be driven into plastic materials and wherein those portions of the flanks which are immediately adjacent the root are parallel to the axis of the screw and the neighboring sections of the flanks have constant radii of curvature all the way to the radially outermost sections which extend radially outwardly to the crest of the thread and the cross-sections of which in a plane including the longitudinal axis of the thread are straight lines. Accordingly, each of those sections of the flanges which are located within the aforementioned imaginary cylinder the diameter of which matches the pitch diameter of the thread includes two parts, namely a radially inner part having an infinitely large radius of curvature and a radially outer part the cross-section of which in a plane including the axis of the thread constitutes a portion of a circle, i.e., a curve having a constant radius of curvature.
U.S. Pat. No. 4,799,844 (granted Jan. 24, 1989 to Chuang for "Elliptical Thread Design" discloses a screw structure with a root portion extending between adjacent thread turns and having a curvature defined by a portion of an ellipse for providing improved stress reduction during periods of severe loading. The patented screw structure is designed primarily to take up compressive stresses and to form part of a down hole percussion drilling tool. The flank angle of the thread on the drilling tool of Chuang is greater than 45.degree.. The configuration of the thread on the patented drilling tool is selected for the specific purpose of withstanding compressive stresses and is not satisfactory when a threaded member or part is to resist pronounced (constant and/or dynamic) tensional stresses. The patentee considers it advisable to avoid the provision of flanks having large radii of curvature adjacent the root of the thread.
U.S. Pat. No. 4,040,327 (granted Aug. 9, 1977 to Otaki for "High Fatigue Screw Threads" discloses a symmetric screw thread system with a flank angle of 70.+-.2.degree. and with flank sections which are parallel with the axis in the region adjacent to the root of the thread. Such threads are incapable of standing pronounced tensional and/or dynamic stresses. The transition zones between the axially parallel radially inner sections and the outer sections of the flanks have extremely small radii of curvature.
Applicant is further aware of an article on page 48 of the German-language publication entitled "Machinenmarkt" (dated Oct. 25, 1988) which discusses a tool for non-cutting shaping (molding) of threads in hard metallic materials. Pages 31 to 34 of the German-language publication entitled "Kunststoffberater" (published in June 1983) contain an article by Jurgen Onasch which discusses a universal thread for a sheet metal screw. The article by P. Dietz and J. Blechschmidt on pages 836-838 of the German-language publication entitled "Maschinenmarkt" (published in 1985) discusses the influence of configuration of the thread upon the ability of the threaded part to withstand stresses. The article proposes to increase the flank angle in order to enhance the distribution of stresses. Pages 213 to 218 of the German-language publication entitled "Konstruktion" (published in 1986) contain an article by M. Weck and F. Fortsch which discusses a computerized process for shaping screw threads.